hertz's experiments. 211 



iiK'clianicnl lone that would be exerted at the place if a unit ({uautity 

 of electricity were there. It is not a force itself at all; it is only a 

 descrii)tion of the condition of the medium at the ])lace which makes 

 electricity there tend to move. The air near the earth is in such a con- 

 dition that everything immersed in it tends to move away from the 

 earth with a finxe of about 1.2C dynes for each cul)ic centimeter ol the 

 body, i. c. each cubic centimeter tends to moxc with a force of 1.26 

 dynes. Kow, the condition of the air that causes tliis is never described 

 as volume (brce existinj>' at the ])lace, tlioui^h we do describe the corre- 

 sponding" condition of the a'ther as electric force existing' there; and as 

 volume force existing would be a very objectionable jjescription of the 

 condition of the air, when being at different pressures at various levels, 

 it tends to make bodies move with a force i)ro])orti()nal to their vol- 

 ume, so electric force existing is a very objectionable description of the 

 condition of the fether, whatever it is, that tends to make bodies move 

 with a force in proportion to their electric charges. We know more 

 about the structure of the air than we do about the ;ether. We know 

 that the structure of the air that causes it to act in this way is that 

 there are more molecules jumping about in each cubic centimeter near 

 the earth than there are at a distance, and we do not know yet what 

 the structure of the a4her is that causes it to act in this remarkable 

 way; but even though we do not know the nature of the structure, we 

 know some of its effects, by means of wiiich we can measure it, and we 

 can give it a name. Although we know very little indeed about the 

 structure of a piece of stressed india rubber, yet we can m<^asure the 

 amount of its stress at any pLice, and can call the india rubber in this 

 peculiar condition "^stressed india rubber." As a matter of fact, we 

 know a great deal more about the peculiar condition of the a4her that 

 we describe as "electric force" existing, than we do about the 

 "stressed india rubber;"' and there is every reason to suppose that the 

 structure of the a'ther is, out of all comparison, more simple than that 

 of india rubber. 



When sound-waves travel through the air, they consist of compres- 

 sions followed by rarefactions, and between them the i>rcssure varies 

 from i)oint to point, so that here we have travelling Ibrward a stnu'ture 

 the sanu; as that of the air near the earth, and waves of sound might 

 be described as consisting ol' a succession of positive and negative 

 "volume Ibrces" travelling forward in tlu^ air; tliis ibrm of expression 

 would no doubt be objectionable, but still if all we knew a1)out the 

 properties of the air near the earth was that it tended to make bodies 

 move away from the earth with a force proportional to their volume, it 

 is quite likely that this condition of affairs near the earth might have 

 been described as the existence of a "volume forc<'" near the eaith, 

 and when it was discovered that this action was due 1o a medium, the 

 ail-, it would have been <pute natural to describe this state of the air as 

 "volume force" existing in it: an(l then wlien waves of sound were ob 



